Expert: Keith Patton - 10/14/2009

Question
Hi Keith,
Thank you for offering your time to answer questions. I was wondering how Indonesia has been hit again with another big earthquake after 5 years? Is this still an aftershock from then? I do not know much about earthquakes. However, I would have thought the earthquake 5 years ago would have released a lot of pressure. The reason I wonder and worry is because I am born and raised in California and after the Northridge earthquake I left California and swore never to go back. I decided after the "big one" and things are back to normal I would move back if I am still alive by then. Now I do not think I would if 5 years later another huge one could happen. I am scared to death of earthquakes and cannot sleep if I am somewhere prone to quakes. However, I want to move back someday.


Answer
Katelin:

The first thing you have to realize it the gigantic magnitude of the forces involved.  We are talking continents here...do you fly?  Look down on a forest from the air, remember how small you are compared to a tree?  Not consider the amount of energy it takes to move a rock the size of your car.  Now magnify that to move the whole state of California, then the whole of North America.  Get the picture?

In California, everything on the east side of the San Andreas fault is moving west. On the west side of the fault, the Los Angeles and south San Francisco side the plate is moving northeaast.
So the two plates are grinding into each other.  Farther north along the Oregon and Washington coast, the two plates overlap, with the west plate actually going under the N. American plate.  This is called subduction and is why there are volcanos there and not around LA.

There are volcanos like Mt. Lassen in California due to the same reason.

Rock has finite strength and when the rocks of the two plates lock up as they slide past each other (what we call a transform fault) pressure builds up.  When the strength of the rock is exceeded movement along the fault occurs in a spastic jump.  Aerial photos along the San Andreas fault show hills offset by as much as a mile from movement along the fault over many times in the past.

After the initial quake, aftershocks occur due to "readjustment" as the rocks settle into a new state of equilibrium as weak rocks fail letting the stresses settle on stronger more competent rocks.  Then the whole process starts over again as the pressure starts to build again.

In LA the picture is more complicated as instead of one fault you have many and a quake can occur along any one of the many faults in the complex or swarm of faults that are braided around and through the Los Angeles area.  San Francisco has more than a few too, but the big one is the San Andreas fault (I went to school in Virginia and we had a T-shirt that said "San Andreas is not our fault"...bad pun.)   What makes the earthquakes worse in some areas is the fact that basins like LA and SF are filled with sediment that is saturated with water at a certain depth.  The bedrock moves causing the quake, and the overlying sediment shakes like a block of jello.  That is what causes most of the massive damage.  Buildings are not built to withstand horizontal movement, they are designed to withstand vertical stress.  Earthquake buildings take lateral movement into account and are strengthened to withstand that.  If you live in an area that is build on less sediment and closer to bedrock you will minimize the shaking of your structure after the initial quake.  This is why areas like the Marina district of SF suffered so much, it was a manmade landfill area and was very much like pudding.  Interstitial water (water between sand grains in the fill material) bubbled out when the material shook causing it to settle, much like wiggling your toes when standing on the wet sand at the beach causes your feet to start to sink into the sand.

Yes, they are calling for a big one but the 1991 quake was not it.  That was just an intermediate quake.  I don't know what the periodicity of the big quake events are suppose to be out there, but the USGS web site should have more information.  http://www.usgs.gov/